1. Field of the Invention
The present invention relates to a device for injecting a mixture of air and fuel into a combustion chamber of a turbomachine, such as an airplane turboprop or turbojet.
2. Description of the Related Art
An injection device of this type generally includes primary and secondary swirlers that are disposed downstream from an injector, coaxially therewith, each of which delivers radial streams of air downstream from the fuel injector so as to mix the air and the fuel that is to be injected and then burnt in the combustion chamber. The flow of air from the primary swirler is accelerated in a Venturi that is interposed between the two swirlers. A bowl of frustoconical shape is mounted downstream from the swirlers and guides the air/fuel mixture that enters into the combustion chamber.
Each swirler has a plurality of substantially radial vanes defining between them curved or inclined air-passing channels so as to impart rotary motion to the air about the axis of the swirler, thereby forming a swirling stream of air.
The head of the fuel injector is axially engaged in a centering ring that is mounted to slide radially in a bushing situated upstream from the swirlers so as to accommodate differential thermal expansions between the various parts in operation. The ring includes axial air-passing orifices that open out radially inside the primary swirler. The orifices in the ring are situated on a circumference of a diameter that is smaller than the diameter of circumference passing via the radially inner trailing edges of the vanes of the swirler.
In the prior art, the number of orifices in the ring and the number of vanes in the primary swirler are determined independently of each other. Nevertheless, the air streams leaving the orifices in the ring disturb the swirling air stream delivered by the primary swirler, thereby giving rise to turbulence in the swirling air stream that can give rise to soot and coke being deposited on the inside surface of the Venturi.
This deposit may impede injection of the air/fuel mixture into the chamber and may give rise to local hot points inside the chamber, thereby in particular encouraging the emission of harmful gases such as nitrogen oxides (NOx).